Abstract
Physiological responses, including autonomic and neuroendocrine changes, are evoked during threat aversive to maintenance of homeostasis. Although the relevance, the neurobiological mechanisms involved in etiology and control of these responses are poorly understood. The bed nucleus of the stria terminal (BNST) is a limbic structures localized in the rostral prosencephalon involved in autonomic, neuroendocrine and behavioral responses to stress. A recent study from our group demonstrated an inhibitory influence of the endocannabinoid signaling in the BNST, acting via local CB1 receptor, in tachycardiac response to acute restraint stress, without affecting the blood pressure increase and cutaneous vasoconstriction responses. It has been reported an inhibitory action of BNST CB1 receptor in local glutamatergic neurotransmission. An important mechanism of nitric oxide (NO) formation in the central nervous system is mediated by activation of the NMDA glutamatergic receptor, which in turn activates the neuronal isoform of the NO synthases enzyme (nNOS). The NO has several signaling mechanisms, but the main is mediated by stimulation of cyclic guanosine monophosphate (cGMP) formation that in turn activates protein kinase G. Although above evidence, a possible involvement of local glutamatergic and nitrergic signaling in regulation of cardiovascular function by BNST endocannabinoid neurotransmission has never been investigated. Therefore, a first hypothesis to be tested in the present study is that the control of cardiovascular responses during an acute stress session by BNST CB1 receptor is mediated by local glutamatergic neurotransmission, acting via NMDA receptors and dependent of NO formation and activation of the soluble guanylate ciclase and protein kinase G. The neural circuitry involved in control of cardiovascular responses to stress by the BNST is poorly understood. However, it was identified projections, possible GABAergic, from the BNST to the lateral hypothalamus (LH). A recent study demonstrated an inhibitory influence of the LH in tachycardiac response to acute restraint stress. Thus, considering that control of cardiovascular responses to stress by BNST endocannabinoid signaling is mediated by a modulation of local glutamatergic neurotransmission, a second hypothesis to be tested in the present study is that the endocannabinoid formation within the BNST during stress decrease the local glutamate release, which in turn reduces the activation of local GABAergic neurons that project to LH, causing a lesser inhibition of neurons in the LH. This effect would mediate the inhibitory influence of BSNT endocannabinoid signaling in tachycardiac response to stress. Therefore, the aims of the present study are: 1) to investigate the involvement of the NMDA receptor/NO/cGMP/protein kinase G signaling pathway in control of cardiovascular responses to acute restraint stress by the CB1 receptor within the BNST; 2) to characterize the role of GABAergic neurotransmission in the LH in control of cardiovascular responses to acute restraint stress in rats; and 3) to evaluate the involvement of LH GABAergic neurotransmission in control of cardiovascular responses to acute restraint stress by the CB1 receptor in the BNST. (AU)
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